The color white is defined by the International Commission on Illumination (CIE).
Organic light-emitting diodes (known by the acronym “OLED”) are fast-developing optoelectronic components.
Whereas a “conventional” light-emitting diode consists of inorganic semi-conducting materials, an OLED consists of a stack of layers of organic materials, among which is included at least one electroluminescent, fluorescent or phosphorescent layer. This results in a fabrication technology which is much simpler and less expensive to implement.
OLEDs emitting a white light can find application in domestic lighting and in the back-lighting of flat screens (liquid-crystal or filtered screens).
To generate a white light it is necessary to combine at least two emitters of different colors. Consequently, white OLEDs comprise at least two distinct electroluminescent layers, exhibiting two different emission wavelengths, or else a single layer containing a mixture of two electroluminescent materials also exhibiting two different emission wavelengths.
FIG. 1 shows a flat band diagram of a white OLED with two spatially separate fluorescent emitters. Depicted from left to right are an anode A, a hole-transport layer HTL, a red electroluminescent layer ELR, a blue electroluminescent layer ELB, an electron-transport layer ETL and a cathode C. The rectangles represent the forbidden bands of the various materials, lying between the HOMO (“highest occupied molecular orbital”) and LUMO (“lowest unoccupied molecular orbital”) levels. The electroluminescent layers are composed of a matrix and of a fluorescent dopant; the dashed lines represent the HOMO and LUMO levels of the dopants, the continuous lines those of the matrices. The reference NV indicates the level in vacuo.
The chromatic coordinates of the two emitters must be situated, in a CIE1931 diagram, on two points joined by a segment passing through the region of said diagram corresponding to white (in the case of three or more emitters: the polygon obtained by joining the points representing the emission colors of the emitters must contain at least one portion of the white region of the CIE1931 diagram).
However, the emission spectrum of a white light source that is to be used as lighting is not characterized satisfactorily by just the coordinates of the corresponding point in the CIE1931 chromatic space. It is also necessary to take account of a parameter known as the color rendering index (CRI). A satisfactory CRI requires that white be formed on the basis of emitters whose representative points in the CIE1931 diagram are very remote. In particular, at least one of these emitters must generate a light exhibiting a very saturated color. Generally, this entails the blue emitter, thereby making it possible to minimize its proportion of luminance while ensuring emission exhibiting the desired chromatic coordinates. This choice makes it possible to limit the electrical stress imposed on the blue organic emitters, which exhibit a smaller lifetime than those emitting light of greater wavelength and thus constitute the factor which limits the lifetime of white OLEDs. However, it is also necessary to consider that, for one and the same luminance level, the wider the forbidden band of the blue emitters, and therefore the more saturated the blue color of the light they emit, the shorter their lifetime. Consequently it is necessary to find a compromise, not always satisfactory, between CRI and lifetime.
“Blue” emitters or emitters “with wide forbidden band” mean electroluminescent materials exhibiting a forbidden band lying, by way of indication, between 2.48 and 3.26 eV, corresponding to emission wavelengths of between 380 and 500 nm. In a manner known per se, these materials generally consist of a non-luminescent matrix (or host material) containing a fluorescent or phosphorescent dopant. When one speaks of the forbidden band of an emitter, one is referring to the dopant; the forbidden band of the matrix is slightly wider.
The lifetime of an organic material with wide forbidden band is limited by several physical and physicochemical phenomena.
Firstly, these materials oxidize easily on account of their LUMO level being close to the void level.
Secondly, numerous blue emitters are stable in an ionic form and deteriorate rapidly in the ionic form of opposite sign. Thus, these materials need to be used to conduct electrons mainly (case of FIG. 1), or holes mainly; the presence of minority carriers or of excitons (this being necessary to produce light emission) causes a degradation of electrochemical nature.
Thirdly, it is known empirically that these materials conduct carriers relatively poorly, and therefore undergo significant electrical stress.
These phenomena, which lead firstly to a progressive decrease in luminance over time, and then to a failure of the diode, also occur when blue emitters are used alone (in blue OLEDs), but in this case it is possible to compensate for them, at least up to a certain point, through a progressive increase in the supply voltage. On the other hand, in the case of intimate association of two or more emitters, the difference in their lifetimes causes a progressive and difficult-to-compensate modification of the color point, that is to say of the chromatic coordinates of the emitted light.
Furthermore, in the case of intimate association of two or more emitters, the excitons generated in the emitter of large forbidden band ELB have a tendency to diffuse toward the emitter of smaller forbidden band ELR, reducing the luminance of the blue emitter ELB, which must therefore be subjected to a higher electrical stress.
These problems are not solely specific to white OLEDs, but affect all OLEDs comprising a blue emitter associated with an emitter of narrower forbidden band.
It is known to separate the electroluminescent layers of a white OLED (more generally, with two or more emitters) with an electron-blocking and/or hole-blocking layer intended to control the color point by modifying the distribution of the carriers in the device. A blocking layer is defined by a less (more) energetic LUMO (HOMO) level than the LUMO (HOMO) level of the adjacent layer on the cathode (anode) side when dealing with an electron-blocking (hole-blocking) layer. A barrier height or difference of electronic level is considered to have a blocking effect provided that it is greater than 0.3 eV. This solution is described in the article by CH. Kim and J. Shinar “Bright small molecular white organic light-emitting devices with two emission zones” Appl. Phys. Lett., Vol 80, no. 12 (2002) pp 2201-2203. It exhibits the drawback of reducing the conductivity of the diode, since precisely the blocking layer constitutes a barrier to the transport of the carriers. Consequently, at equal luminance, a higher potential difference must be applied across the terminals of the OLED, thereby limiting the gain in terms of lifetime.
It is also known from the prior art to form the emitter layer as two adjacent sub-layers of distinct host materials doped with the same electroluminescent dopant, the whole being dubbed D-EML for Double Emissive Layer (M. Ben Khalifa et al. “Efficient red phosphorescent organic light emitting diodes with double emission layers”, J. Phys. D: Appl. Phys. 41 (2008) 155111). This organic junction exhibits the advantage of associating a hole-conducting host on the anode side and an electron-conducting host on the cathode side. Thus the carrier recombination zone is situated at this junction, and limits the diffusion of the excitons toward adjacent layers external to the D-EML. However, it is very difficult, or even impossible, to apply this solution in the case of a diode associating two or more emitters, since this makes it necessary to considerably increase the number of evaporated layers (4 for two emitters and 6 for three), and on the other hand since the sub-layers in contact of the two or three D-EMLs must ensure non-negligible conductivity both for the holes and for the electrons.
It is also known from the prior art to introduce an exciton blocking layer provided that fluorescent and phosphorescent emitters are associated in one and the same diode. See the article by Sung Hyun Kim, Jyongsik Jang and Jun Yeob Lee “High efficiency phosphorescent organic light-emitting diodes using carbazole-type triplet exciton blocking layer” Applied Physics Letters 90, 223505, 2007. Indeed, phosphorescent emitters are beneficial for low-luminance applications, where they exhibit very high efficiencies. But as there is currently no phosphorescent emitter with wide forbidden band exhibiting a sufficient lifetime, it is not possible to produce an entirely phosphorescent white light-emitting diode. Hence the benefit of associating a blue fluorescent emitter and a phosphorescent emitter with greater wavelength. However, if the precaution were not taken of separating the phosphorescent emitter and the fluorescent emitter with an exciton blocking layer and these two emitters were directly juxtaposed, the triplet excitons of the phosphorescent material could diffuse toward the fluorescent material, where they would de-excite in a non-radiative manner. An exciton blocking layer typically exhibits a thickness of 3-10 nm, and a LUMO (HOMO) level which is lower (higher) than that of the dopant of the phosphorescent emitter. The exciton blocking layer, likewise, reduces the conductivity of the OLED, necessitating the use of a high operating voltage which has an unfavorable effect on the lifetime of the emitters.
The documents EP 1 784 056 and EP 1 936 714 disclose OLEDs exhibiting a plurality of electroluminescent layers separated by intermediate exciton blocking layers. The electroluminescent layers consist of one and the same host material containing various dopants; the intermediate layers consist of this same host material, but without doping. As explained above, the exciton blocking layers reduce the conductivity of the OLED, this having an unfavorable effect on the lifetime of the emitters.
The document US 2009/0001875 discloses OLEDs comprising two electroluminescent layers separated by an intermediate hole-conducting layer. Once again, such a configuration can only reduce the conductivity of the OLED, and consequently the lifetime of its emitters.
The documents DE 10 2007 058005 and US 2010/0314648 disclose OLEDs exhibiting three electroluminescent layers separated by two buffer layers, the latter consisting of a mixture of an electron-transporting material and of a hole-transporting material.